A semiconductor device requires a good ohmic contact to permit efficient transfer of electricity from external wires to the device. The metal ohmic contact must be physically and chemically compatible with the semiconductor material of the device. An ohmic contact is usually defined as a metal-semiconductor contact that has negligible contact resistance relative to the bulk or spreading resistance of the semiconductor (Physics of Semiconductor Devices by Sze, Second Edition, 1981).
In wide band semiconductor device such as, for example, silicon carbide devices, limited electrical conductivity of the semiconductor due to low hole or electron concentrations may hinder or even prevent the formation of an ohmic contact. Therefore, the formation of low resistivity and thermally stable ohmic contacts to silicon carbide is one of the most critical stages in device processing. A general approach to this problem and an overview of the main results has been published by Crofton et al. in (Phys. Status Solidi B 202, (1997) 581).
Ohmic contacts to silicon carbide are typically formed by the deposition of transition metals (possibly in combination with other metals) onto heavily doped silicon carbide followed by high temperature annealing. This high temperature of annealing (typically in excess of 900 degrees Celsius) may negatively affect other component parts of the device.